If they can't justify why this is a bug, and why it's worth your time to fix this as opposed to something else, you should be able to revoke their permission to touch your backlog.

I wouldn’t delete any items any backlog items. Much better to always close with a reason. You can make this a first class status in whatever platform like a “Closed - Duplicate”, “Closed - Unneeded”, etc.

This helps with searching for rationale in the future, looking back to similar items, etc.

You can’t stop someone from putting it back but you can close it as duplicate and link it as related.

Edit: also if the person putting work in the backlog is not a core contributor or product you should formalize some other intake process that can triage whether it meets the bar to get in